The present invention relates to a multiple access satellite communication system in which a hub earth station and a plurality of mini-earth stations communicate over common shared channels via a satellite and, more particularly, to a mini-station-to-hub earth station access system.
In a POS (Point-of-Sales) system, banking/financial credit card verification or similar computer communication system, user terminals are connected to mini-earth stations to communicate with a host computer which is connected to a hub earth station. While each mini-earth station transmits information to only the hub station via a satellite over an inbound channel, the hub station sends a response to the information to all the mini-earth stations over an outbound channel. Each mini-earth stations selects only the information meant therefor out of the received signal and transfers it to the associated terminal. Specifically, information is interchanged only between the mini-earth stations and the hub station.
The individual mini-earth stations transmit information to the hub station using time slots which are the divisions of one frame time. The access of each mini-earth station to a time slot may be implemented by any one of three different methods known in the art, as follows.
A first method is a so-called fixed assignment access method which assigns exclusive time slots fixedly to the individual mini-earth stations. Every time a transmission request to the hub earth station occurs, each mini-earth station transfers a packet to the hub station by using the exclusive time slot assigned thereto. This kind of scheme is advantageously applicable when the terminals connected to the individual mini-earth stations output a transmission request constantly. However, when the transmission request occurs irregularly, the method is not desirable from the standpoint of efficient use of time slots.
Generally, satellite channels are lower in quality than wired communication channels and, therefore, bring about transmission errors. It has been customary, therefore, to cause a transmitting station to transmit a CRC (Cyclic Redundancy Check) code or similar error detection code together with data and cause a receiving station to constantly monitor the error detection code to see if packets have been transmitted without errors. On detecting an error in the data of a received packet, the hub station returns a Not Acknowledgement (NAK) signal indicative of a particular time slot associated with the packet with an error to all the mini-earth stations, urging the transmitted mini-earth station to re-transmit. In response to the NAK signal, the mini-earth station of interest re-transmits the data for which the NAK signal is meant by using the exclusive time slot thereof. Although the mini-earth station may have already received data following the data in question, the re-transmission data is sent out prior to the subsequent data. Stated another way, once the re-transmission of data occurs, the exclusive time slot of the mini-earth station is continuously occupied by the re-transmission data until the latter is out. Then, the subsequent data are accumulated in the mini-earth station with the result that the interval between the delivery of data from the terminal to the mini-earth station and the arrival of that data at the hub station is undesirably increased.
A random access method or slotted ALOHA method is a second method and allows a mini-earth station to transmit data by using any time slot every time the data is generated. The problem with this method is that packets sent from a plurality of mini-earth stations are apt to collide in the same time slot. In the event of collision, the hub station returns the NAK signal to all the mini-earth stations indicating that the packets in the time slot of interest were not received correctly. In response to the NAK signal, the mini-earth stations which transmitted the data in collision re-transmit them after the lapse of a period of time which is determined by using a random number, for example. So long as the frequency at which the individual mini-earth stations transmit packets is relatively low and packets are sent at random, this scheme substantially minimizes the need for re-transmission and thereby insures high throughput. Nevertheless, as the frequency of transmission from the individual mini-earth stations increases, the frequency of collision also increases to lower the throughput. When the transmission frequency from the mini-earth stations further increases, even the re-transmitted packets collide degrading throughput to a critical extent.
A demand assignment access method is a third method known in the art. A mini-earth station implemented with this method sends a request for the reservation of the number of slots to be used to a hub station every time a terminal associated with the mini-earth station produces a transmission request. On receiving the reservation request, the hub station assigns time slots which the mini-earth station that sent the request can use, i.e. reserved time slots. Reserved slot assignment information is returned to all the mini-earth stations. This approach is desirable when indidual mini-earth stations send a great amount of data needing a plurality of slots, to a hub station. Even when the transmission frequency from the mini-earth stations increases, this method eliminates the collision of packets particular to the slotted ALOHA method. However, each mini-earth station cannot transmit at all until it receives reserved slot assignment information from the hub station. The demand assignment access method, therefore, needs a longer interval between the transmission of data from a transmitting terminal to its associated mini-earth station and the arrival thereof at a receiving terminal than the fixed assignment access or slotted ALOHA method.
As discussed above, the fixed assignment access method, random access method and demand assignment access method each have advantages and disadvantages. Efforts have heretofore been made to combine these different methods in order to make the most of their advantages. For example, a combined random and demand access method is taught by Fujii et al in a paper entitled "AA/TDMA-ADAPTIVE SATELLITE ACCESS METHOD FOR MINI-EARTH STATION NETWORKS", IEEE Global Telecommunications Conference Record, pp. 42.4.1-42.4.6, December, 1986.
A mini-station implemented by the above-mentioned combined random and demand access method determines the lengths of data to be transmitted to a hub station and thereby classifies them as either short data and long data. Short data has a length smaller than a certain threshold value and can be transmitted in, for example, one time slot, while long data has a length greater than the threshold value and cannot be so transmitted. When long data is fed from the terminal to the mini-earth station, the mini-earth station sends a reservation request to the hub station for reserving the number of time slots which it needs to send the long data. Concerning short data, the mini-earth station sends it to the hub station by using a time slot which is not assigned to itself or any other mini-earth station as a reserved time slot, i.e. a random access slot.
The combined random and demand access method executes a unique re-transmission procedure when short data sent from a certain mini-earth station has collided with short data sent from another mini-earth station. Specifically, when a mini-earth station receives a NAK signal from a hub station after the transmission of data by the random access method, it determines that the satellite traffic is heavy and transmits short data left non-transmitted at the time of arrival of the NAK signal by the demand assignment access method together with long data. The mini-earth station sends re-transmission data by the demand assignment access method also. After an Acknowledgement (ACK) signal has returned from the hub station in reply to the re-transmission data transmitted by the demand assignment access method and all the short data sent before the arrival of the NAK signal, the mini-earth station again begins transmitting subsequent short data to the hub station by the random access method.
As stated above, when the supply of short data to the individual mini-stations increases, the combined random and demand access method inhibits random packet transmission so as to reduce the probability of collision. This, coupled with the fact that the delay ascribable to the second and successive transmissions of the same data is reduced, allows the combined method to be advantageously applied to a case wherein the amount of data to be sent from each mini-station fluctuates noticeably.
On the other hand, the terminals connectable to the mini-earth stations include terminals of the type generating data constantly. Data fed from this type of terminal to the associated mini-earth station should preferably be transmitted to the hub station by the fixed assignment access method, as stated earlier. The combined random and demand access method cannot meet this need. Especially, the combined method of Fujii et al. is not adequate when it is desired to send data from a terminal of the type generating data constantly to the hub station prior to data from the other terminals.